1. Field of the Invention
The present invention relates to an optical apparatus such as a camera having a detector for detecting a visual axis of an observer.
2. Related Background Art
Conventionally, various apparatuses (e.g., an eye camera) for detecting an observation position on an observation surface, i.e., a so-called visual axis, of an observer have been proposed.
Japanese Laid-Open Patent Application No. 2-264632 discloses a visual axis detection apparatus, which irradiates infrared rays onto an eyeball of an observer, and detects a visual axis of the observer by utilizing positions of cornea reflected images and an image of a pupil based on light reflected by the cornea of the eyeball.
FIG. 1 is an explanatory view of the principle of a known visual axis detection method.
Referring to FIG. 1, light sources 13a and 13b comprise, e.g., light-emitting diodes for irradiating infrared rays to which an observer is insensitive. The light sources 13a and 13b are arranged to be substantially symmetrical in the x-direction about the optical axis of a light-receiving lens 12, and illuminate an eyeball 15 of an observer with divergent light. The light sources 13a and 13b are arranged to illuminate the eyeball 15 of the observer from lower positions (i.e., positions offset in the y-direction). Some light components of the illumination light reflected by the eyeball 15 are focused on an image sensor 14 via the light-receiving lens 12. The eyeball 15 has a cornea 16 and an iris 17.
FIG. 2A is a schematic view of an image of an eyeball projected onto the image sensor 14, and FIG. 2B shows an intensity distribution of a signal from an output line of the image sensor 14.
The visual axis detection method will be described below with reference to FIG. 1 and FIGS. 2A and 2B.
Infrared rays emitted by the light source 13a irradiate the cornea 16 of the eyeball 15 of the observer. At this time, a cornea reflected image d formed by some light components of the infrared rays reflected by the surface of the cornea 16 is focused by the light-receiving lens 12, and is formed at a position d' on the image sensor 14. Similarly, infrared rays emitted by the light source 13b irradiate the cornea 16 of the eyeball 15. At this time, a cornea reflected image e formed by some light components of the infrared rays reflected by the surface of the cornea 16 is focused by the light-receiving lens 12, and is formed at a position e' on the image sensor 14.
On the other hand, light beams reflected by end portions a and b of the iris 17 form images of the end portions a and b at positions a' and b' on the image sensor 14 via the light-receiving lens 12. When the rotation angles, .theta., of an optical axis 15a of the eyeball 15 with respect to the optical axis of the light-receiving lens 12 are small, if the x-coordinates of the end portions a and b of the iris 17 are respectively represented by xa and xb, the x-coordinate, xc, of the central position, c, of a pupil 19 is given by: EQU xc.congruent.(xa+xb)/2
On the other hand, the x-coordinate of the middle point between the cornea reflected images d and e substantially coincides with the x-coordinate, xo, of the center, o, of curvature of the cornea 16. For this reason, if the x-coordinates of the positions d and e of the cornea reflected images are respectively represented by xd and xe, and the standard distance between the center o of curvature of the cornea 16 and the center c of the pupil 19 is represented by L.sub.OC, the rotation angle, .theta.x, of the optical axis 15a of the eyeball 15 substantially satisfies: EQU L.sub.OC *sin.theta.x.congruent.(xd+xe)/2-xc (1)
Note that * represents a multiplication in this specification. For this reason, as shown in FIG. 2A, by detecting the positions of the respective feature points (the cornea reflected images and the center of the pupil) of the eyeball 15 projected onto the image sensor 14, the rotation angles .theta. of the optical axis 15a of the eyeball 15 can be obtained.
From formula (1), the rotation angles of the optical axis 15a of the eyeball 15 are given by: EQU .beta.*L.sub.OC *sin.theta.x.congruent.{(xpo-.delta.x)-xic}*pitch(2) EQU .beta.*L.sub.OC *sin.theta.y.congruent.{(ypo-.delta.y)-yic}*pitch(3)
where .theta.x is the rotation angle of the optical axis of the eyeball in the z-x plane, and .theta.y is the rotation angle of the optical axis of the eyeball in the y-z plane. (xpo, ypo) is the coordinate position of the middle point between the two cornea reflected images on the image sensor 14, and (xic, yic) is the coordinate position of the center of the pupil on the image sensor 14. "pitch" is the pixel pitch of the image sensor 14. .beta. is the imaging magnification determined by the position of the eyeball 15 with respect to the light-receiving lens 12, and is obtained as a function of the interval between the two cornea reflected images in practice. .delta.x and .delta.y are the correction terms for correcting the coordinates of the middle point between the cornea reflected images, which include a correction term for correcting an error caused by illumination of the eyeball of a photographer not with collimated light but with divergent light, and a correction term for correcting an offset component associated with .delta.y, that is caused by illumination of the eyeball of the photographer with divergent light from the direction of the lower eyelid. The formulas of the correction terms are disclosed in Japanese Laid-Open Patent Application No. 2-264633.
After the rotation angles (.theta.x, .theta.y) of the optical axis 15a of the eyeball of the observer are calculated, the gazing point (x, y), on the observation surface, of the observer is, for example, given by: EQU x [mm]=m*{.theta.x-(cx*Rp+dx)}/(ax*Rp+bx) (4) EQU y [mm]=m*{.theta.y-(cy*Rp+dy)}/(ay*Rp+by) (5)
Note that the x-direction indicates the horizontal direction with respect to the observer, and the y-direction indicates the vertical direction with respect to the observer. m is a conversion coefficient for converting the rotation angle of the eyeball 15 into a coordinate on the observation surface, and Rp is the diameter of the pupil. ax, bx, cx, dx, ay, by, cy, and dy are gazing point calibration coefficients, i.e., correction coefficients for matching the rotation angles of the eyeball 15 of the observer with the gazing point on the observation surface. Japanese Laid-Open Patent Application Nos. 4-138431 and 4-138432 each disclose an apparatus having a visual axis detection device which can detect whether or not an observer wears spectacles. According to an embodiment of the proposed apparatus, when it is detected that an observer wears spectacles, the position adjustment of a light-receiving optical system, and the like is performed to attain satisfactory visual axis detection.
However, when an observer wears spectacles, the positions of the cornea reflected images and the iris image formed on the image sensor are displaced by the refraction effect of two refraction surfaces of a spectacles' lens (image distortion). In such a case, visual axis detection with high precision cannot be attained by the conventional visual axis calculation method.
Japanese Laid-Open Patent Application No. 5-100147 discloses a camera having a visual axis detection device which includes correction means for correcting or invalidating the visual axis detection result by detecting whether or not an observer wears spectacles. However, the correction means is controlled to limit the functions of the camera for an observer who wears spectacles (e.g., by limiting the visual axis detection area to the central area of the frame or disabling visual axis detection). For this reason, such an observer cannot fully use the functions of the camera, resulting in inconvenience.